The present invention relates generally to a downhole pump, and in one particular embodiment, to a double stroke downhole pump.
Typically, downhole pumps include a hollow plunger sliding up and down within a polished barrel to lift fluids, such as oil and water, to the surface. Usually, the fluid is forced through a check valve located at the bottom end of the plunger on the downstroke of the plunger. On the upstroke, the check valve on the plunger is closed and the plunger lifts the fluid upward. At the same time, more fluid is drawn into a chamber in barrel through a check valve located in the bottom of the barrel. In this operation, the pump is required to lift both the weight of the plunger, a rod string attached to the plunger and the oil.
In other arrangements, shown for example in U.S. Pat. No. 5,314,025 to Priestly and U.S. Pat. No. 6,368,084 to Skillman, the check valves and plunger are arranged such that fluids are pumped on the downstroke of the plunger. However, such pumps do not pump any fluid on the upstroke and are susceptible to gas lock. Moreover, the Priestly and Skillman patents disclose that the oil is forced upwardly through the plunger on the downstroke, which can reduce the capacity of the pump. In addition, Priestly further discloses that various seals are required between the plunger and/or rod string and the pump casing.